A monochrometer is an optical instrument that can select a narrow band of wavelengths of light from a source which contains a broader spectrum. Spectrometers are the combination of a monochrometer and a detector such that the output of a spectrometer is an electrical signal which is proportional to the intensity of light in the selected narrow band. Monochrometers and spectrometers are used in many important commercial and defense applications, some of which include chemical analysis by optical absorption, emission line characterization, thin film thickness analysis, and optical characterization of mirrors and filters.
The optical properties of an unknown material can reveal important information leading to a determination of its composition or physical properties. For instance, spectral analysis of optical emission lines are used to determine the atomic species of gaseous material. A second example is the routine use of optical spectra by the semiconductor industry to determine the thickness of multilayer thin films. These measurements are made with instruments incorporating optical spectrometers. A typical spectrometer is a precision instrument that usually consists of an entrance slit, a prism or grating, a couple of mirrors or lenses, and an exit slit. Lenses would normally be used to focus the light into the entrance slit and from the exit slit onto a detector. To scan through the spectrum, the grating or prism is rotated mechanically. The grating or prism separates the light into its spectral components and these are selected by the exit slit and measured with an optical detector.
The conventional optical spectrometer is a large, expensive, precision instrument. Its quality is characterized by its ability to separate spectral components or in other words, by its resolution. Analytical equipment that incorporates optical spectrometers are by nature expensive and therefore relegated to applications that can justify the expense. While current spectrometers perform their function well, broader application of optical measurement techniques would be achieved with a small and less expensive alternative.